Voltage and current transients generated during switching of high voltage circuit breakers are of increasing concern for the electric utility industry. These concerns include both power quality issues for voltage-sensitive customer loads, and excessive stresses on power system equipment. Conventional solutions for reducing switching transients include circuit breaker pre-insertion devices such as resistors or inductors and fixed devices such as arresters and current limiting reactors. While these solutions provide some degree of mitigation, they require additional equipment, increase costs and are accompanied by reliability concerns.
A superior approach to reduced switching transients is synchronous switching, sometimes referred to as point-on-wave switching. Synchronous switching is performed by a synchronization device, which—upon receiving a close (or trip) command—delays the switching of the circuit breaker by a selected period of time such that current inception (or contact separation) coincides with a certain point on the voltage wave, known to reduce switching transients. For instance, for connecting shunt capacitor banks, the circuit breaker is preferably switched in at voltage zero crossing.
FIG. 1 illustrates schematically, partly in a block diagram, partly in a circuit diagram, a portion of a circuit breaker station in which a control arrangement based on the above approach is implemented. The circuit breaker station 101 comprises, for each phase, a plurality of circuit breakers 104, 105, 106, voltage measuring devices 107, 108 for measuring voltages of bus bars 102, 103, current measuring devices 109, 110, 111 for measuring currents at terminals of the circuit breakers 104, 105, 106, and, for each circuit breaker, a circuit breaker position indicating device 112 for indicating the position of the circuit breaker. Such position indication devices are often an integral part of the respective breaker operating mechanism. However, for reasons of clarity only one circuit breaker position indicating device 112 is illustrated in FIG. 1. The control arrangement comprises a central control system 113 for the overall control of the circuit breaker station 101, the central control system 113 being connected to the voltage measuring devices 107, 108, the current measuring devices 109, 110, 111, and the circuit breaker position indicating devices 112 to obtain measured voltages, currents, and circuit breaker positions from there. The connections are only schematically indicated by reference numerals 114, 115, 116 at the central control system end.
For each of the circuit breakers 104, 105, 106, there is provided a synchronization device 117 connected to the central control system 113 via connection 118 to obtain switching instructions, and to the respective circuit breaker 104, 105, 106 via connection 119 to control the operation thereof. The synchronization device may e.g. be the Switchsync™ Controller commercially available from ABB. Further, each of the synchronization devices 117 is connected to one or several voltage measuring devices 107, 108, to at least some of the current measuring devices 109, 110, 111, and to the circuit breaker position indicating device 112 to obtain a measured voltage, measured currents, and a measured circuit breaker position from there. The connections are indicated by reference numerals 120, 121, 122, 123, wherein connection 120 has a voltage selector for selecting which of the bus bar voltages that should be forwarded to the synchronization device 117. Note that for reasons of clarity only one synchronization device 117 is illustrated in FIG. 1. Similarly, for reasons of clarity only the connections 118-123 to and from the illustrated synchronization device 117 are indicated in FIG. 1.
Each of the synchronization devices 117 is arranged to determine a switching delay for the circuit breaker 105, to which it is connected, based on the obtained voltage, currents, and circuit breaker position, and to switch the circuit breaker 105 after the determined delay in response to receiving a switching instruction from the central control system 113. Depending on whether the circuit breaker 105 closes or opens in the switching operation, and depending on the type of load (shunt capacitor banks, shunt reactors, transformers, transmission lines, etc.), the switching operation should be performed such that current inception (or contact separation) coincides with a certain point on the voltage wave. As an example, the synchronization devices 117 may be configured to close at voltage zero crossing.